Not really sure if you can inflate a mealworm to make it float. What I usually do is put on a white power egg and then thread the mealworm on the hook. A thought that just came to mind is inject some trout oil scent into the mealworm, that might help it float since oil will float to the surface. I've done this with nightcrawlers before, just inject them with some garlic oil, you get the best of both worlds. The nightcrawler floats and slowly releases scent.